This disclosure relates to the field of nuclear magnetic resonance (NMR) well logging apparatus and methods. More specifically, the disclosure relates to methods and apparatus for NMR well logging that can make accurate measurements of NMR properties of formations surrounding a well at greater speed of motion of the well logging instrument without degradation of the measurements as a result of motion of the well logging instrument.
NMR-based measurement of the fractional volume of pore space (porosity) of subsurface formations, which is substantially independent of the rock mineral composition (matrix) of the subsurface formations, has been widely accepted. In complex environments, where the matrix properties are not well known, NMR measurements may provide more accurate porosities than other well logging instruments used to determine porosity. In addition, NMR transverse relaxation time (T2) distributions may provide additional information about bound-fluid porosities and reservoir quality. Notwithstanding the superior quality of NMR porosity measurements, NMR well logging remains a niche service. One reason that has hindered NMR well logging from becoming a more widely used service for determination of subsurface formation properties is the relatively slow logging speeds that may be used with NMR instruments known in the art compared to that of other porosity tools.
NMR-determined porosities and T2 distributions are adversely affected by speed of motion of the well logging instrument along a wellbore. Speed effects are more problematic with NMR measurements because of the fact that NMR measurements take a relatively long time (e.g., seconds) compared to other logging tool measurements. Logging speeds for NMR well logging instruments known in the art are relatively slow, e.g., typically in the range from 300 to 900 feet per hour depending on the logging environment and the specific application.
There exists a need for NMR well logging instruments and measurement techniques that can measure accurate porosities and T2 distributions at much greater speeds, for example, 1800 feet per hour as may be used with gamma-gamma density porosity and neutron porosity well logging instruments.